To increase system capacity, Universal Mobile Telecommunication System (UMTS) based wireless communication systems will evolve to a packet data system. All services, including voice services, will be supported by the packet data system. The packet data system will use an Orthogonal Frequency Division Multiple Access (OFDMA) air interface for transmitting data packets between Node Bs and User Equipments (UE). The OFDMA air interface comprises a set of orthogonal sub-carrier frequencies, i.e., resources, which are partitioned in terms of frequency and/or time into reverse link and forward link resources.
In the packet data system, the reverse link resources may be dedicated resources or shared resources. For purposes of this application, the term “dedicated resources” should be construed to mean resources which are dedicated to a particular UE for use for an entire data session, such as a Voice over Internet Protocol (VoIP) call, and the term “shared resources” should be construed to mean resources which are not dedicated resources, i.e., the resources may be used by multiple UEs during a data session.
Transmissions over the reverse link resources may be contention or non-contention based. When the transmission is contention based, multiple UEs may be simultaneously transmitting over a same reverse link resource. When the transmission is non-contention based, multiple UEs are not permitted to simultaneously transmit over a same reverse link resource. For non-contention based transmissions, a scheduler at the Node B is used to allocate the reverse link resources such that multiple UEs are not using a same reverse link resource at the same time. Unless otherwise stated, all transmissions described herein are non-contention based transmissions.
FIG. 1 depicts a chart 100 illustrating a method of allocating reverse link resources to a UE for a VoIP call in accordance with the prior art. At time t0, the UE is in an idle state when data arrives in its buffer. The data may include call setup information and/or data packets, such as VoIP call setup signaling messages and speech packets. At time t1, the UE transmits a selected preamble to a Node B over a non-synchronized Random Access CHannel (RACH), wherein the non-synchronized RACH is non-synchronized in the sense that it is not time synchronized with the Node B. The preamble indicates to the Node B that the UE needs resources in order to transmit its data. The preamble is randomly selected by the UE from a set of preambles, wherein each preamble is a unique signature sequence which have low cross correlation with each other. For example, the set of preambles includes Walsh codes or sequences which have low cross correlation with each other.
Note that preambles transmitted over the non-synchronized RACH are contention based transmissions. The low cross correlation characteristic of the preambles allows a Node B receiving two or more different preambles simultaneously over the non-synchronized RACH to distinguish one preamble from another. If two or more UEs transmit a same preamble simultaneously on the non-synchronized RACH, then a collision may occur. In such a situation, neither preamble may be successfully received by the Node B. The UEs would have to wait some predetermined or random time interval before re-transmitting the same or another preamble.
At time t2, the Node B detects the preamble and allocates shared resources to the UE for transmitting a scheduling request message (SRM). Note that when shared resources are allocated, such allocation of resources is usually for some fixed time duration or transmission time interval (TTI) which expires before the call ends. The SRM is a resource request indicating information about the data, among other things, such that reverse link resources may be properly allocated by the Node B for the transmission of the data. For example, the SRM may indicate information regarding an amount of data and data type.
At time t3, the Node B transmits to the UE an acknowledgement and a resource assignment message over an Acquisition Indicator CHannel (AICH) or other control channel. The acknowledgement indicates that the preamble was received and the identity of the preamble that was received (which can be used by the UE to determine the intended recipient of the acknowledgement). The resource assignment message indicates the shared resources allocated to the UE for sending the SRM.
At time t4, the UE receives the acknowledgement and resource assignment message. The UE selects a SRM from a set of SRMs. The set of SRMs include subsets of (one or more) SRMs, wherein each subset is associated with different categories of data. For example, one subset might be associated with 400 bits of speech, whereas another subset might be associated with 600 bits of video. Each SRM is a signature sequence having low cross correlation with other SRMs. The SRM (or subset) is selected based on which category the data matches.
At time t5, the UE transmits the selected SRM using the allocated shared resources indicated in the resource assignment message. At time t6, the Node B receives the SRM and allocates shared resources to the UE based on the category of the received SRM, wherein the allocated shared resources are sufficient for the transmission of the data. At time t7, the Node B transmits over a control channel, such as a grant channel, a scheduling grant indicating the allocated shared resources and the UE to which the shared resources are being allocated.
At time t8, the UE receives the scheduling grant which allows the UE to enter an active state in which the VoIP call (or data session) is activated. At time t9, the UE uses the allocated shared resources to transmit its data. At time t10, the Node B receives the data.
While the VoIP call (or data session) is active, the UE receives more data (e.g., speech packets) in its buffer at time t11. The UE selects another SRM and waits until a synchronized RACH is available, wherein the synchronized RACH is synchronized in the sense that it is time synchronized with the Node B. At time t12, the synchronized RACH becomes available and the UE transmits the SRM. The SRM is transmitted at some transmit power based on an interference level broadcasted by the Node B (for an associated cell or sector), a desired signal to interference ratio (SIR) at the Node B and a path loss estimated from a pilot signal transmitted by the Node B. If the power used to transmit the SRM is insufficient, the SRM may not be received properly by the Node B. In such a case, the UE would have to wait some random or predetermined time interval before transmitting another SRM.
Note that when the UE is in an active data session, it uses the synchronized RACH to transmit the SRM. The SRM transmissions over the synchronized RACH are contention based transmissions (unlike when the UE is in the idle state and using allocated shared resources to transmit the SRM, i.e., non-contention based transmission). If another UE transmits the same SRM simultaneously on the synchronized RACH, then a collision may occur and the UE would have to wait some random or predetermined time interval before transmitting another SRM.
The SRM is received by the Node B at time t13. Shared resources are allocated to the UE for transmission of its data based on the SRM. At time t14, the Node B transmits a scheduling grant over a control channel, such as a grant channel, indicating the allocated shared resources. The UE receives the scheduling grant at time t15 and transmits its data at time t16 using the allocated shared resources. At time t17, the Node B receives the data.
When the UE is in an active data session, there exists a latency period from when the UE receives data in its buffer at time t11 to when the UE transmits the data at time t16. The latency period is due, in large part, to delays associated with the UE waiting for the synchronized RACH to become available, collisions occurring on the synchronized RACH and/or insufficient initial transmit power for the SRM. Such delays may result in latency period being upwards of 50-60 ms. For time sensitive data applications, such as VoIP calls, latency periods upward of 50-60 ms are unacceptable. Accordingly, there exists a need to reduce the latency period which occurs when the UE is in an active data session.